Transportation vehicles, such as automobile and truck bodies, are treated with multiple layers of coatings which enhance the appearance of the vehicle and also provide protection from corrosion, scratch, chipping, ultraviolet light, acid rain and other environmental conditions. Basecoat/clearcoat finishes for automobiles and trucks have been commonly used over the past two decades, in a “wet-on-wet” application, i.e., the transparent clear coat is applied to the basecoat before the colored base coat is completely cured.
Minimum performance requirements for these basecoat/clearcoat finishes include high levels of acid etch resistance, intercoat adhesion, repair adhesion, substrate adhesion, adhesion to windshield bonding adhesives, scratch and mar resistance, chip resistance, humidity resistance, weatherability, and the like. Basecoat/clearcoat coatings and/or the individual components thereof must also be capable of providing excellent visual appearance characterized by a high degree of gloss, distinctness of image (DOI), and smoothness. The latter requirements are particularly important for clearcoat compositions.
Scratch and mar resistance has proven to be a particularly difficult performance property to achieve relative to the balance of other required performance and appearance properties. Scratch and mar resistance typically refers to a coating's ability to resist scratching from mechanical abrasions caused by car wash bushes, tree limbs, keys, fingernails and the like. Scratch and mar resistance are particularly important attributes for clearcoats, as the clearcoat is the exterior most coating in the basecoat/clearcoat system.
In addition to providing the foregoing performance and appearance parameters, the various coating components must be easy to apply in a manufacturing environment.
Finally, any coating composition that is intended for use in a composite basecoat/clearcoat system must be compatible with a wide variety of other coating compositions. For example, a coatings manufacturer may not formulate a basecoat composition for use solely with one particular primer or clearcoat composition. Furthermore, in many automotive paint shops, the clearcoat supplier may not have control over the basecoat formulas that are used. Also, in what is becoming an increasingly desirable two-tone painting operation, two different types of basecoats, i.e., a waterborne basecoat for the main body panel and solventborne basecoat for the lower accent area, are now being used beneath the clearcoat. In such cases where the clearcoat supplier has no control over the basecoat formula, it is particularly desirable to have compatibility with a wide range of basecoat types, including both waterborne and solventborne basecoats.
Compatibility and ease of use with many commercially available coating compositions is thus a necessity for the individual components of a composite basecoat/clearcoat coating system. A successful clearcoat composition will be compatible with both waterborne and solventborne basecoat compositions, as well as medium and high solids versions thereof. This compatibility must exist regardless of the differences in film-forming technology. “Compatible” as used herein refers to a combination of two or more individual coating components which provides acceptable levels of the previously discussed performance, appearance and application requirements of composite basecoat/clearcoat systems.
However, certain basecoat formulations present particular compatibility challenges for the clearcoat coating manufacturer. For example, it has been found that if a waterborne basecoat composition containing free amine is applied to a substrate, followed by the application of a traditional solventborne clearcoat composition comprising a monomeric melamine as a crosslinking resin, upon simultaneous curing of both layers, the resulting cured film appears “wrinkled”. This appearance is undesirable and lacks commercial value. It has been found that clearcoat compositions containing polymeric melamines, particularly those containing high imino polymeric melamines, can provide good appearance and wrinkle resistance over amine containing waterborne basecoats. As a result, the current commercially available clearcoats utilize polymeric melamine as the crosslinking entity. However, it has also been found that polymeric high imino aminoplast resins can lead to poor scratch and mar resistance, and unacceptable adhesion over baked solvent or waterborne basecoats, which are now more popularly practiced in vehicle assembly plants where basecoats are sprayed, such as in a two-tone painting operations, over a wet primer in the primer spray booth and baked in the former primer only ovens.
Thus, the challenge for the coatings manufacturer is to provide coating compositions, especially clearcoat compositions, which provide all of the necessary performance, appearance and application properties discussed above but which are further compatible with a wide array of commercially available coating compositions. More particularly, it would be advantageous to retain or improve the performance, appearance and application parameters of prior art clearcoats but without the basecoat compatibility issues discussed above.
Notwithstanding the foregoing, the prior art has failed to provide clearcoat coating compositions which possess the necessary balance between performance, appearance and application requirements but are compatible with a wide variety of basecoat formulations, especially the most challenging basecoat formulations.
Accordingly, it is an object of this invention to provide a coating composition which can be used as a clearcoat over a wide variety of basecoat formulations, including both waterborne and solventborne basecoats, including waterborne basecoats containing free amines, to provide multilayer coated articles which have a substantially wrinkle free appearance, good intercoat adhesion, excellent scratch and mar resistance, and provide desired levels of durability and etch resistance.